A type of such control apparatuses set forth above is designed to carry out current feedback control to thereby adjust an actual value of at least one controlled variable of a rotary machine to a command value. A typical control apparatus of this type carries out triangular-wave comparison PWM (Pulse Width Modulation) control for driving switching elements of an inverter as an example of power converters.
Specifically, the triangular-wave comparison PWM control is designed to calculate a substantially sinusoidal command voltage for each phase winding of a three-phase motor as an example of rotary machines; this command voltage is required to match an actual current flowing through each phase winding with a desired periodic command current.
The triangular-wave comparison PWM control is designed to compare the sinusoidal command voltage for each phase winding with a triangular carrier wave. Based on the result of the comparison, the triangular-wave comparison PWM control is designed to individually switch on and off each of a plurality of bridge-configured switching elements of an inverter based on the result of the comparison. This modulates an input voltage, such as a DC voltage, to the inverter into an AC (Alternating Current) voltage to be applied to each phase winding of the three-phase motor.
Specifically, adjustment of the on and off durations, that is, the duty (duty cycle) of each of the bridge-configured switching elements under the triangular-wave comparison PWM control matches the AC voltage to be applied to each phase winding with the command voltage therefor. This matches the actual current flowing through each phase winding to a desired periodic command current. The actual current flowing through each phase winding works to generate, as the at least one control variable, a torque corresponding to the desired command current for each phase winding.
When the command voltage is greater in amplitude the half of the inverter input DC voltage under the triangular-wave comparison PWM control so that the inverter is driven in an overmodulation mode, the output voltage of the inverter may include higher harmonic contents with large amplitudes. These higher harmonic contents may adversely affect on the following capability of the actual current flowing through each phase winding with respect to the corresponding command current. These adverse effects are due to the fact that the system of the triangular-wave comparison PWM control is designed assuming that any value of the output voltage of the inverter can be set as a value of the command voltage.
In order to address these adverse effects, Japanese Patent Application Publication No. 2008-228419 discloses a method of driving each of the bridge-configured switching elements under model predictive control. The method under the model predictive control is designed to predict a d-axis current value and a q-axis current value for each of a plurality of switching modes (drive modes) of a bridge-configured inverter for driving a three-phase motor.
The method is also designed to determine an optimum one of the plurality of switching modes. The optimum one of the plurality of switching modes allows the deviation of a d-axis command current value from the predicted d-axis current value and the deviation of a q-axis command current value from the predicted q-axis current value to be minimised; these d- and q-axis command current values are examples of a controlled variable of the three-phase motor. The method is further designed to drive the bridge-configured inverter according to the determined one of the plurality of switching modes.
Alternative examples of the method under the model predictive control are disclosed in Japanese Patent Application Publication No. 2006-174697 and in Hirokazu KOBAYASHI, Shinji DOKI, and Shigeru OKUMA, “Current Control System using Model Predictive Control with Integral Procedure”, the 2007 Tokai-Section Joint Conference of the Eight Institutes of Electrical and Related Engineers.